Conventional methods for producing a metal powder include atomization methods. The atomization methods include a water atomization method in which a metal powder is obtained by injecting a high-pressure water jet into a stream of molten metal and a gas atomization method in which an inert gas is ejected instead of a water jet.
In the water atomization method, a stream of molten metal is divided into powdery metal (metal powder) using a water jet ejected from a nozzle and the powdery metal (metal powder) is cooled with the water jet, whereby an atomized metal powder is obtained. On the other hand, in the gas atomization method, a stream of molten metal is divided into powdery metal using an inert gas ejected from a nozzle. Thereafter, the powdery metal (metal powder) is usually cooled in such a manner that the powdery metal is dropped into a water tank or flowing water drum placed under an atomizer, whereby an atomized metal powder is obtained.
In recent years, for example, motor cores for use in electric or hybrid automobiles have been required to have low iron loss from the viewpoint of energy saving. Hitherto, motor cores have been manufactured by stacking electrical steel sheets. Recently, motor cores manufactured from a metal powder (electromagnetic iron powder) with a high degree of freedom in shape design are attracting attention. In order to produce such a motor core with low iron loss, a metal powder with low iron loss needs to be used. In order to allow a metal powder to have low iron loss, the non-crystallization (amorphization) of the metal powder is probably effective. However, in order to obtain a non-crystalline metal powder by an atomization method, crystallization needs to be prevented by rapidly quenching the metal powder in a high-temperature state including a molten state.
Therefore, several methods for quenching a metal powder have been proposed.
For example, Patent Literature 1 describes a method for producing a metal powder in such a manner that the cooling rate until solidification is set to 105 K/s or more when the metal powder is obtained by cooling and solidifying molten metal by scattering the molten metal. In a technique described in Patent Literature 1, the above cooling rate is obtained in such a manner that the scattered molten metal is brought into contact with a coolant stream generated by swirling a coolant along the inner wall of a cylinder. The flow velocity of the coolant stream generated by swirling the coolant is preferably 5 m/s to 100 m/s.
Patent Literature 2 describes a method for producing a rapidly solidified metal powder. In a technique described in Patent Literature 2, a coolant is supplied to a cooling container having an inner surface that is a cylindrical surface from the outside edge of the upper end of a cylindrical portion of the cooling container in a circumferential direction and is dropped in such a manner that the coolant is swirled along the inner surface of the cylindrical portion, a layered swirl coolant layer having a space in a central portion thereof is formed by the centrifugal force due to the swirl, and molten metal is supplied on to the inner circumferential surface of the swirl coolant layer and is rapidly solidified. This allows a high-quality rapidly solidified metal powder to be obtained with good cooling efficiency.
Patent Literature 3 describes an apparatus for producing a metal powder by a gas atomization method. The apparatus includes a gas jet nozzle for dividing molten metal flowing down into molten droplets by ejecting a gas jet and also includes a cooling cylinder including a coolant layer swirling down along the inner surface thereof. In a technique described in Patent Literature 3, molten metal is divided in two stages, with the gas jet nozzle and the swirling coolant layer, respectively, whereby a fine rapidly solidified metal powder is obtained.
Patent Literature 4 describes a method for producing fine amorphous metal particles in such a manner that molten metal is supplied into a liquid coolant, a vapor film is formed in the coolant so as to cover the molten metal, the molten metal is brought into direct contact with the coolant by disrupting the vapor film formed such that boiling is caused by spontaneous nucleation, the molten metal is rapidly cooled to be amorphized while the molten metal is being torn by the pressure wave of the boiling, and the fine amorphous metal particles are thereby obtained. The vapor film covering the molten metal can be disrupted by ultrasonic irradiation or in such a manner that the temperature of the molten metal supplied to the coolant is adjusted such that, when the molten metal is in direct contact with the coolant, the interfacial temperature is not lower than the spontaneous nucleation temperature nor higher than the minimum temperature of film boiling.
Patent Literature 5 describes a method for producing fine particles in such a manner that the temperature of a molten material is set prior to supplying the molten material into a liquid coolant in the form of droplets or a jet stream such that the temperature of the molten material is not lower than the spontaneous nucleation temperature of the liquid coolant and a molten state is kept when the molten material is brought into direct contact with the liquid coolant, the difference in relative speed between the molten material supplied in a stream of the liquid coolant and the liquid coolant stream is adjusted to 10 m/s or more such that a vapor film formed around the molten material is forcedly disrupted and boiling is caused by spontaneous nucleation, and atomization and solidification by cooling are caused, this enabling a conventionally and otherwise difficult material to be atomized and amorphized.
Patent Literature 6 describes a method for manufacturing a functional member. The method includes a step of obtaining polycrystalline or non-crystalline, homogeneous functional fine particles free from segregation in such a manner that a raw material obtained by adding a functional additive to a material serving as a matrix is melted, is supplied into a liquid coolant, and is atomized by vapor explosion and the cooling rate is controlled during solidification by cooling and also includes a step of obtaining the functional member in such a manner that the functional fine particles and fine particles of the matrix are used as raw materials and are solidified.